This invention relates to an apparatus for automatically baking semiconductor wafers in order to improve the adhesion between a mask material and the wafer.
In the manufacture of semiconductor devices, prior to photoengraving, etching, or ion implantation, baking treatment is carried out in order to increase the adhesion between the mask material and the wafer. There are various processes for performing baking treatment, including sheet treatment in which wafers are treated one at a time and batch treatment in which wafers are inserted into a cassette prior to treatment and the entire cassette is inserted into a baking furnace so that a large number of wafers can be treated at a single time. This invention relates in particular to an apparatus for use in batch treatment.
Conventionally, during baking treatment, as shown in FIG. 1, a silicon wafer (hereinunder referred to as a "wafer") is inserted into one of a number of slots 2 formed in a Teflon cassette 1 (hereinunder referred to as a "TF cassette") and undergoes baking treatment in a baking furnace 10 like that shown in FIG. 2 in cross section.
In FIG. 2, a TF cassette 1 containing a wafer 5 is mounted on a rack 22 for cassettes in the furnace interior 21. An air intake port 23 is provided below the rack 22, and a heater 16 is installed inside the air intake port 23. An air blower 13 is installed beneath the furnace interior 21, near the heater 16. The air blower 13 is driven by a motor 11 which is connected to the air blower 13 by an output shaft 12. The air blower 13 is positioned in the middle of a duct 15 which extends upwards to the top of the furnace 10 and communicates with the upper portion of the furnace interior 21. A filter 14 is provided at the upper portion of the duct 15. Air passes from the duct 15 into the furnace interior 21 via the filter 14 and an unillustrated air discharge port.
A temperature-sensing element 17 for measuring the temperature in the furnace interior 21 is installed on the inner wall of the furnace 10 near the racks 22 for cassettes. The furnace interior 21 is surrounded by thermal insulation 25. An opening is formed in one of the walls of the baking furnace 10, and a door 18 with a handle 19 is mounted on this opening. At the bottom of the baking furance 10 is a control panel 20 for regulating the treatment conditions within the furnace interior 21. On the bottom surface of the baking furnace 10 are mounted casters 30 and adjustable bolts 31 which serve as adjustable-length legs and enable the height of the furnace 10 to be raised or lowered as necessary.
During baking treatment, air (indicated by the arrows 24) which is heated by the heater 16 rises upwards from the bottom of the duct 15, passes through the filter 14, and is blown through the air discharge port into the furnace interior 21 by the air blower 13. Wafers 5 within the TF cassette 1 which is mounted on one of the racks 22 within the furnace interior 21 are treated by the hot air thus supplied. The hot air passes through the air intake port 23 provided at the bottom of the furnace interior 21, flows past the heater 16, and is reheated.
During baking treatment, the temperature within the furnace interior 21 is generally about 70-250 degrees C., and treatment is usually performed for about 20-60 minutes. In order to perform treatment using a baking furnace 10, usually a wafer 5 which was treated in a previous step is inserted into a polypropylene cassette (hereinunder referred to as a "PP cassette"), transported to the baking furnace 10, and then transferred to a TF cassette 1 immediately prior to baking treatment. The door 18 of the baking furnace 10 is manually opened, a prescribed number of TF cassettes 1 are placed on the racks 22 using a special jig, the door 18 is manually closed, and baking treatment is performed for a prescribed length of time. The hot TF cassettes 1 are then removed from the furnace interior 21 using a jig, and the wafers 5 are allowed to cool in the TF cassettes 1 for about 20-40 minutes until they reach room temperature. Next, the wafers 5 are transferred either manually or by a special automatic transferring machine to a PP cassette or to another TF cassette, depending on what the next step in manufacture is.
After baking treatment, unless a wafer 5 is cooled below a prescribed temperature, the treatment conditions in the succeeding steps and the quality of the wafer 5 are affected. Furthermore, it is necessary to allow a TF cassette 1 to cool after baking treatment because when it is in a hot state, the pitch of its slots 2 for wafers 5 is greater due to thermal expansion than the pitch of a cold PP cassette, and due to the difference in pitch it is difficult to transfer wafers 5 from a hot TF cassette to a cold PP cassette by an automatic transferring machine.
Conventionally, during cooling, wafers 5 are left in a hot TF cassette until both are cool. However, due to the heat capacity of the TF cassette, it takes a great deal of time for the wafers 5 to cool. Cooling a wafer 5 while it is still in a hot TF cassette is therefore an inefficient and time-consuming method.
A conventional baking furnace also has the drawback that it requires an extremely large door 18 to enable an operator to safely and easily insert and remove cassettes from the furnace interior 21. Because of the size of the door 18, a great deal of heat escapes from the furnace interior 21 each time the door 18 is opened, causing a drop in the temperature of the furnace interior. This results in a waste of heat and a loss of time while the furnace interior 21 is reheated in order to once again reach the appropriate temperature for baking treatment.
The opening and closing of such a large door also creates air turbulence which introduces dust and other foreign matter into the furnace interior 21. Also, as the length of time for which the door is opened depends on how quickly the operator can insert the cassettes, the fluctuation in temperature within the furnace varies from batch to batch, and it is difficult to obtain uniform treatment conditions.